In this competitive renewal of our R01 grant, we propose to extend the human cell models that we have built to investigate the molecular mechanisms of our findings on the relation of Alzheimer's disease (AD) to APP and ApoE. Familial AD represents a small subset of AD cases that are caused by inherited fully penetrant mutations in the APP, Presenilin1, or Presinilin2 genes. Sporadic AD, in contrast, is associated primarily with genetic variations in ApoE as the highest known genetic risk factor followed by the microglial gene TREM2. How the ApoE4 genotype leads to AD with A? deposition and Tau pathology, however, is incompletely understood, and the cell biological and genetic interactions between ApoE4, TREM2, and A? remain unclear. In the previous funding period, we have established human stem cell-based neuronal cell models to interrogate the effects of APP mutations and ApoE variants. We made several key observations: (i) ApoE activates a DLK/MAP2K7/ERK pathway in neurons in an isoform-specific manner inducing a c-Fos-dependent transcription of APP; (ii) ApoE also induces the formation of synapses via the same pathway but independent of c-Fos but instead dependent on CREB; (iii) ApoE induces the activation of additional signaling pathways via thus far uncharacterized receptors; (iv) the APPSwe mutation leads to increased Abeta production and Tau phosphorylation but seemingly paradoxically also induces the formation of more synapses in young neurons. Based on these observations, we propose that ApoE acts as a signaling molecule in the brain which activates specific neuronal receptors, that this action, at least in part, collaborates with a function of APP in synaptic information processing by neurons, and that ApoE4 and APP mutations promote neurodegenerative processes by enhancing the efficacy of the normal functions of ApoE and APP. Recent data have convincingly demonstrated that the microglial gene TREM2 can act as an ApoE receptor. It suggests that ApoE4 acts directly on microglia to predispose the brain to a neuroinflammatory insult mediated by microglial TREM2. This is an intriguing result that is persued now by many groups. Instead, given our past observations, we here propose to focus our investigation on direct effects of ApoE on neurons. We consider neuronal functions as non-exclusive and complementary to the role in microglia. Specifically, we propose to characterize additional pathways that our preliminary data show are activated by ApoE and identify the neuronal ApoE receptors that mediate the identified MAPK signaling and the potentially other pathways. We will also include microglia and mouse neurons in vivo in this analysis. We further propose to investigate the mechanisms of how the endogenous Swedish APP mutation leads to increased synapse formation in young human neurons and whether Abeta aggregates in these cultures with and without experimental perturbation. Finally, we propose to investigate whether ApoE and APP mutations may operate in a synergistic manner to influence synapse formation, Abeta aggregation and Tau phosphorylation.